KR100939448B1 - Electrolytic cell with gas diffusion electrode - Google Patents

Abstract

Described herein is a membrane electrolytic cell comprising an anode compartment and a cathode compartment, at least one of the two compartments comprising a gas supply electrode and wherein a porous planar element overlaps between the membrane and the gas supply electrode. Chemically corrosive electrolytes flow downward across the porous planar element under the action of gravity. Planar devices consist of plastic devices that withstand corrosive operating conditions: Perfluorinated plastics such as ECTFE, PTEFE, FEP, and PFA are highly hydrophobic but are preferred to use them. If the gas supply electrode is a cathode and the gas contains oxygen, the gas crosses the cathode compartment upwards to minimize the risk of hydrogen accumulation. Cells equipped with an oxygen cathode are particularly advantageous for sodium chloride electrolysis.

Electrolytic cells, porous planar devices, plastic materials, gas diffusion electrodes, anode compartments, cathode compartments, sodium chloride electrolysis.

Description

Electrolytic cell with gas diffusion electrode {Electrolysis cell with gas diffusion electrode}             

In electrolytic cells, chlor-alkali electrolysis for the production of hydrochloric acid gas and caustic soda or potash, mainly water electrolysis for producing hydrogen, the corresponding acids and bases from sodium sulfate, eg For example, several industrial processes have been carried out, such as salt electrolysis to obtain caustic soda and sulfuric acid, metal deposition (typically copper and zinc). The essential problem with all these processes is electrical energy consumption, which typically accounts for a significant portion of the overall manufacturing cost. Since electrical energy is characterized by an ever-increasing cost in all geographic areas, it is clear that it is important to reduce its consumption in the electrochemical process described above.

The energy consumption in the electrochemical process is mainly dependent on the cell voltage: cell design thus reducing the resistance drop in the cell structure and electrolyte, using more catalytic electrodes, for example by reducing the gap between the electrodes. The reasons for trying to improve this are too obvious.

In the conventional chlor-alkali process, sodium chloride solution or less frequently potassium chloride solution is fed to the anode containing cell, whereby chlorine gas is released at the anode, while at the cathode sodium hydroxide (potassium hydroxide when potassium chloride is supplied) At the same time hydrogen is released. In the most advanced type of cell, caustic soda present near the cathode is fed to the anode region by a cationic membrane consisting of perfluorinated polymer containing negatively charged groups such as sulfonic and / or carboxylic acid groups. Separated from the sodium chloride solution. Such membranes are commercially available from several companies, among them DuPont / USA, Asahi Glass and Asahi Chemical / Japan. The design of this type of cell has been studied in depth and it can be asserted that the technology is currently in the best condition with regard to energy consumption. An example of this type of design is provided in WO 98/55670. However, analyzing the manufacturing costs of chlorine and caustic soda obtained with this advanced cell type, the impact of energy consumption is still significant. For this reason, a common element is to use a gas electrode, specifically a cathode that is supplied with oxygen (self or oxygen rich air, or simply air except carbon dioxide content) as a replacement for the hydrogen emitting cathode used in the techniques discussed above. An improved series of proposals has been made.

Chlorine-alkaline electrolytic cells comprising a cathode fed with an oxygen containing gas exhibit essentially lower electrical energy consumption than in a typical prior art. The reason for this fact is, among other things, due to its thermodynamic nature, where the two cells, the conventional cell and the cell containing the oxygen cathode, are characterized by the following different overall reactions:

-Conventional battery

2NaCl + 2H ₂ O → 2NaOH + Cl ₂ + H ₂

Cells with oxygen cathodes

2NaCl + H ₂ O + ½O ₂ → 2NaOH + Cl ₂

In fact, the voltage of a conventional membrane cell supplied with a current density of 4 kA / m 2 is about 3 V, whereas the voltage of a cell equipped with a membrane and an oxygen cathode is about 2 to 2.2 V when operated under the same operating conditions. Is observed. As can be seen from this, about 30% of the electrical energy savings are achieved (typically secondary to the fact that no hydrogen used as fuel is produced). However, at present, the electrolytic cell in which the oxygen cathode is inserted is not used industrially. The reason for this situation lies in the structure imposed on the oxygen cathode and the requirements imposed on the operating conditions to ensure good overall efficiency. In short, the oxygen cathode consists of a porous support, preferably a conductive support, on which a microporous layer formed by an assembly of electrocatalyst particles mechanically stabilized by a binder resistant to operating conditions is applied. The layer may comprise additional films comprising particles and binders which are preferably conductive but nonelectrocatalytic. The porous support may be made of mesh, various perforated sheets, carbon / graphite fabrics, carbon / graphite paper or sintered materials. Electrodes of this type and related fabrication processes are described in US Pat. No. 4,614,575.

When the electrode as described above is used as an oxygenated cathode in chlor-alkali electrolysis at a position parallel to the cationic membrane, at a position in direct contact with the membrane or at a suitable gap of 2-3 mm from the membrane. However, caustic soda produced by the reaction of oxygen on the electrocatalyst particles must be released somehow to avoid progressively filling the micropores of the layer. In fact, when such a charge occurs, oxygen reaching the catalyst particles acting as the reaction site can no longer diffuse through the pores. The release of caustic soda formed from the cathode electrocatalyst particles can occur in essentially two ways: in the direction of the membrane if the cathode is located parallel to the membrane or with a certain gap from the membrane, or the cathode is in the membrane itself And in contact with the membrane may occur in the direction of oxygen backspace from the side facing the membrane.

In the former case, a liquid film with a thickness of 2 to 3 mm is formed, which is typically maintained while circulating upwards to extract the caustic soda produced by the cell and to remove the heat naturally generated by the reaction and finally The caustic soda concentration can be adjusted within a predetermined range to extend the useful life of the ion exchange membrane. This situation establishes a pressure gradient between caustic soda and oxygen on both sides of the cathode, which actually acts as a separating wall. This gradient can be positive (the pressure of caustic soda is higher than the pressure of oxygen), in which case the gradient increases from top to bottom under the action of a hydraulic head. Conversely, the gradient may be negative (the pressure of oxygen is higher than that of caustic soda), in which case the gradient decreases from top to bottom under the action of the hydraulic head of caustic soda. Using commercially available materials and known fabrication procedures, it is possible to obtain cathodes (expressed as water columns) that can withstand pressure differences up to about 30 cm. This is because, in order for the oxygen cathode to function optimally, the height of the cell designed to accommodate the oxygen cathode cannot exceed 30 cm. If the height is larger, the voids are completely filled with caustic soda when the difference is positive, and a full flooding of the cathode occurs. When the difference is negative, the loss of oxygen into the caustic soda is large. This is a serious problem for certain scale electrolysis plants, because the total number of small dimensions cells must be very large and the additional costs for auxiliary devices (electrical connections, conduits, pumps) are enormous. It should be taken into account that conventional types of industrial cells equipped with hydrogen emitting cathodes are typically 1 to 1.5 m in height. In order to overcome the above inconvenience, it has been proposed to use a structure in which the cathode is about 2 to 3 mm away from the membrane and the overall height is 1 to 1.5 m, but the cell is subdivided into a number of subunits, each about 30 cm in height. Such a design is incompatible with industrial applications, with significant complexity for the connection conduits between the various subunits, and ultimately with operational complexity and cost. Another structure is that described in US Pat. No. 5,693,202. This provides a design in which the cell maintains a unitary structure and the cell is equipped with an oxygen cathode divided into strips. The oxygen pressure supplied to each strip is automatically controlled using a hydraulic head of caustic soda through a bubbling system. This type of cell overcomes the complexity of the subunit bulkhead design described above, but presents inherent complexity with regard to the need to ensure the sealing by hydraulic and pneumatic pressure of each strip. In addition, special procedures are used for shutdown and start-up in order to avoid loss of pressure compensation at this temporary stage due to lack of oxygen supply. Another solution is exemplified in EP 0150018 A1, which may have a diaphragm or ion exchange membrane interposed between the anode and the cathode and has a falling film of liquid that wets the electrode where the gas release reaction occurs. It is about. Therefore, EP 0150018 A1 does not actually solve the above problem, but rather releases the generated gas bubbles from the reaction liquid in which the gas bubbles are formed, as clearly stated in the opening part of the present specification. It's about making things happen. Difficulties associated with releasing gas bubbles from the liquid are inherently manifested by anode and cathode pressure changes, vibrations, partial blocking of the electrode under the action of attached bubbles, and finally an increase in resistance, which are detrimental to the ion exchange membrane. This is because electrolyte electroconductivity is clearly reduced in the presence of gas. Thus, in EP EP 0150018 A1, the fact that the electrode is completely covered with a liquid film of varying thickness is a process to consider, the formation of gas bubbles and their release from the liquid phase, the diffusion of gas and the electrode surface. It is not a matter of direct relevance, as it is not its consumption in Essau (as is seen above, which is typical of oxygen supplied cathodes). Another proposal is described in EP 1033419 A1 and WO 01/57290, which have a fixed gap from the membrane and are suitable for chlor-alkali electrolysis of membranes equipped with oxygen cathodes. Batteries are described. Caustic soda is fed to the top of the cell and flows downward in the gap defined by the membrane and cathode. In this case the flow rate of caustic soda is very high and a porous layer is introduced between the membrane and the cathode to limit this to the actual level of interest. In addition, oxygen is also supplied in some excess to the top of the cell and released from the bottom with caustic soda. Such a device can dynamically cancel the hydraulic head of caustic soda and maintain a very low and constant level of pressure difference between caustic soda and oxygen, thus achieving ideal operating conditions for the oxygen cathode. According to EP 1033419 A1, the porous layer is characterized by hydrophilicity: in practice, therefore, the plastic material is excluded and, on the one hand, very resistant to reaction environments which become corrosive by the presence of trace amounts of peroxides. Strong, but very hydrophobic, perfluorinated plastic materials, on the other hand, are especially so. Therefore, only the hydrophilic metal or its oxide can be used to prepare the porous layer of the present invention. Nevertheless, these materials are characterized by the specific release of metal ions absorbed by the ion exchange membrane and, therefore, deterioration in performance, in particular in terms of battery voltage and Faraday efficiency, when contacted with concentrated boiling caustic soda. . The only metal that is truly exempt from this inconvenience is silver, and its surface is protected under operating conditions by an impermeable, very poorly soluble oxide: However, the widespread use of silver in electrolytic cell structures is costly and therefore almost unsupported in the industry. It is clear that we do not receive.

In the latter case, when operated using an oxygen cathode in direct contact with the membrane, caustic soda release is the side of the cathode opposite to the side facing the membrane, as described, for example, in US Pat. No. 6,117,286. Only in the direction of the gap occupied by oxygen at. In this case, a series of questions arise as listed below:

Caustic soda, forcedly flowing across the cathode, tends to fill the voids and impede oxygen diffusion. To avoid this inconvenience, the cathode is divided into strips and a portion of the caustic soda formed is released between the membrane and the cathode strip by interposing a hydrophilic porous element such as in EP 1033419 A1 cited above. It has been suggested that it can.

Caustic soda released on the oxygen side has a unique tendency to wet the backside of the cathode to form a continuous film, impeding oxygen diffusion. In order to prevent this detrimental action, the cathode backing needs to be strongly hydrophobic, which can lower the surface electrical conductivity, which complicates the electrical contact required to supply the current.

Since the forced circulation occurs instead in the case of the former of the oxygen cathode, the concentration of the product caustic soda is necessarily generated by the reaction and cannot be controlled under certain limits. The concentration value of the product caustic soda is about 37 to 45%, and this amount depends on the operating conditions such as the amount of water carried across the membrane, the amount and current density depending on the type of membrane, and the temperature and concentration of the alkali chloride solution.

Commercially available ion exchange membranes inevitably degrade when contacted with caustic soda having a concentration of at least 35% even for a relatively short time. Therefore, since the amount of water transported is known to increase as the alkali chloride concentration decreases, it has been proposed to operate a cell with an oxygen cathode in direct contact with the membrane to operate with a dilute solution of alkali chloride. Nevertheless, too low a concentration of alkali chlorides limits the flexibility of the operation allowed by these factors, because the membrane efficiency is poor, the oxygen ratio in chlorine increases and the operating life of the anode can be shortened. For this reason, as a further means, it has been proposed to saturate oxygen with water vapor at a temperature similar to the operating temperature of the cell; By spreading water vapor across the pores of the cathode, the caustic soda concentration can be reduced to a value close to the acceptable value for the membrane. However, this action is only partially effective because part of the water vapor is absorbed by the caustic soda released from the back of the cathode.

It is an object of the present invention to provide an electrolytic cell structure equipped with a gas diffusion electrode without the disadvantages of existing technology. Given the industrial relevance of chlor-alkali electrolysis, a cell suitable for this application will be mentioned below, and the present invention further relates to another electrochemical process, for example electrolysis of alkaline sulphate (containing sulfate in the anode compartment). Solution and supply hydrogen to the counter gas diffusion anode, hydrogen peroxide production (alkali solution to the cathode compartment and oxygen to the counter gas diffusion cathode), and electrical energy generation (hydrogen and oxygen supplied using an alkaline fuel cell) It is apparent that the gas diffusion electrode can be advantageously applied to the supply of an alkaline electrolyte to two compartments in which both are provided.

The cell structure of the present invention is a porous planar element interposed between oxygen cathode compressed by ion exchange membrane, cathode and membrane and intersected by downward flow of caustic soda as is known in the art. Material of the porous planar element is selected from the group of hydrocarbon plastic materials, for example polyethylene, in particular high density polyethylene, preferably perfluorinated plastic materials (for example ethylene-chlorotrifluoro Ethylene ECTFE, polytetrafluoroethylene PTFE, tetrafluoroethylene-hexafluoropropylene FEP, perfluoroalkoxy polymer PFA), which have high mechanical resistance at high temperatures and high concentrations in the presence of trace peroxides Has a fairly chemical inertness against caustic soda and thus a membrane There are characteristic differences does not deleteriously pollution. All of these materials are very hydrophobic, so their use is very contrary to that suggested by the prior art, where hydrophilicity is an essential property.

According to a preferred embodiment, a further feature of the present invention is that an oxygen-containing gas is supplied to the bottom of the cell: in this way, an area where there is no flow since the rising gas flow replaces hydrogen being formed in small quantities as a byproduct of the oxygen consumption reaction. The risk of accumulation in the can be avoided. Hydrogen appears when the electrolytic cell operates at high current densities, resulting in a longer operating life of the cathode as a result of the inevitable natural decline of catalytic activity.

The method of obtaining this result of high industrial relevance will become apparent in the following detailed description of the invention with reference to the accompanying drawings.

1 is a side view of the battery of the present invention.

2 is a front view of a battery of the present invention according to a preferred embodiment.

3 is a front view of a battery of the present invention according to another preferred embodiment.             

It is known to those skilled in the art that industrial electrolyzers and fuel cells preferably consist of a plurality of elementary cells, which are hydraulically and electrically connected so that they are compressed together to form a modular arrangement; This electrical connection can be of the monolo or bipolar type. In addition, the battery of the present invention is also suitable for constituting one of the repeating elements of a modular electrolytic cell or a fuel cell. Referring to FIG. 1, the battery 1 of the present invention is formed by two shells, an anode shell 2 and a cathode shell 7, in the case of a chlor-alkali electrolytic cell, which is preferably titanium and nickel, respectively. It is composed. The two shells are mutually secured by tie-rods or jacks not shown in the figures or by any other system of the prior art, and the two compartments, the anode compartment and the cathode compartment, It surrounds the cation membrane 16 which acts as a separator.

Cationic membranes used for chlor-alkali, specifically sodium chlorine electrolysis, are perfluorinated with sulfonic acid groups (sides facing the anode 3) and carboxylic acid groups (sides facing the cathode 10) in the main chain. It consists of a polymer. Membranes of this type, characterized by low internal ohmic resistance and high current densities, typically even from 3 to 5 kA / m 2, are suitable for Nafion ^R , Flemion ^R and Dupont, Asahi Glass and Asahi Chemical, respectively. It is provided under the trade name Aciplex ^R. The peripheral sealing necessary to avoid the release of chlorine, oxygen, sodium chloride solution and caustic soda into the external environment is ensured by the peripheral gasket 8. The anode shell 2 contains an anode 3 and is based on a titanium sheet with an opening, for example a platinum group metal or a mixed oxide thereof, and also containing an oxide of a valve metal (especially titanium). It consists of a stretched or perforated sheet provided with an electrocatalyst layer for releasing chlorine from. The anode 3 is fixed to the shell 2 by a support 4 which transfers current from the wall of the shell itself to the anode. Cathode current distributor consisting of metal foil with openings, for example metal mesh, elongated or perforated sheets, preferably made of silver or generally silver coated nickel, stainless steel or nickel alloys for optimum electrical contact 11 is fixed to the cathode shell 7 via a support 12 to facilitate current transfer between the shell itself and the surface of the distributor. The current distributor is preferably a thicker first conductive foil provided with an opening, in which a second foil with a smaller opening is overlapped, where the second foil is in contact with the oxygen diffusion cathode. Between the distributor 11 and the membrane 16, the oxygen supply cathode 10 and the hydrophobic planar element 9 of the present invention are inserted into the current distributor side and the membrane side, respectively. The oxygen cathode is made by applying an electrocatalyst particle stabilized with a suitable binder to a porous conductive support as is known in the art. Catalysts of electrocatalyst particles are platinum group metals, mainly platinum, oxides, sulfides thereof, more generally their chalcogenides, pyrochlore (particularly ruthenium pyrochlore), silver and gold. An interesting analysis of scientific knowledge on this subject is described in book Electrochemical Hydrogen Technologies, edited by H. Wendt, Elsevier, 1990, Cap. 3, "Electrocatalysis of the Cathodic Oxygen Reduction", K. Wiesener, D. Ohms. These catalysts can be mixed arbitrarily and used as bulk powders, and the addition of graphite powder arbitrarily for the dual purpose of increasing the cross-conductivity in the bed and reducing the amount of catalyst used provides an optimal compromise between performance and cost. Is achieved. The latter purpose is, for example, Cabot Corporation under the trade names Shavinigan Acetylene Black (hereinafter referred to as SAB) or Vulcan XC-72, which are well known to those skilled in the art. It is also possible to achieve by using a catalyst supported on a conductive material that does not have catalytic activity, such as partially optionally graphitized carbon, such as commercially available from Corp.).

Cathodes and related fabrication methods suitable for operation using oxygen and in which the above materials have been inserted are described in patent documents such as US Pat. Nos. 4,614,575 and 5,584,976.

Accurately selecting the type of hydrophobic planar element (pore volume fraction, average pore diameter size, thickness) and caustic soda flow rate can achieve the following results known in the art: that is, gravity causes Equalizes the pressure of the caustic soda flowing in direction and significantly reduces the positive or negative pressure difference present between the caustic soda and oxygen (which makes the oxygen cathode structure insignificant) and the caustic soda in contact with the membrane. By adjusting the concentration within an optimum range of 30 to 35%, the performance of the caustic soda flow can be removed better, resulting in better performance and longer membrane life.

The shape of the planar element can vary widely. As a non-limiting example, a foam, preferably a foam having continuous bubbles, a mattress consisting of coils in which the wire is knitted, a flat mesh formed by layers of intersected and overlapping wires, a flat mesh made of woven wires, protruding Mention may be made of a mesh of wires shaped to make a surface with regions and depressions. The planar element may consist of only one of these members or of an assembly of overlapping members.

When the shells 2 and 7 having various components are firmly tightened, if the support 12 of the current distributor is deflected and elastically imparted, close contact between the membrane-plane element-oxygen cathode-current distributor The compression force necessary to ensure can be secured mechanically. Alternatively, the required elasticity is achieved after the support 12 is rigid, after a resilient member such as a conductive mattress consisting of a coil or a wavy layer is added to the planar element-oxygen diffusion cathode-current distributor assembly. Can be. The required compressive force can finally be obtained while maintaining a higher pressure inside the anode shell 2 than inside the cathode shell 7. This higher pressure pushes the membrane towards the assembly of the hydrophobic planar element-oxygen cathode-current distributor (in this case the support 12 is rigid), which makes effective contact across the entire range of the various interfaces. During operation, the positive and negative poles of the current generator are connected to the anode shell 2 and the cathode shell 7, respectively, and fresh sodium chloride solution is supplied to the nozzle 5, and the consumed sodium chloride solution and the product chlorine are nozzles (6). ), A new caustic soda solution is injected through the distributor 13, for example a perforated tube, to traverse downwardly the planar element 9 in the longitudinal direction, and at the injected caustic soda and cathode 10 The caustic soda solution obtained under the influence of the oxygen reaction consisting of the mixture formed by the resulting caustic soda is discharged from the nozzle 15.

The present invention, as a first innovative feature, is characterized in that the porous planar element inserted between the membrane and the oxygen cathode consists of a highly inert plastic material, preferably a plastic material of the perfluorinated type, characterized by high hydrophobicity, and in a preferred embodiment, the second As an innovative feature, the lower nozzle 17 (or, in some cases, the same nozzle 15 used to release the product caustic soda) in an excess of 10 to 20% relative to the amount consumed by the total current supplied to the cathode. Since it is supplied to, oxygen (or oxygen-containing gas) flows upward, and excess is discharged from the upper nozzle 14.

Materials considered in the prior art for the manufacture of planar devices are materials of the hydrophilic type: wettability by caustic soda is required to prevent the oxygen bubbles generated by leakage through the cathode voids from permanently adhering to the inner surface of the planar device. It is believed that this property was introduced because it is believed. In this environment, it is apparent that the current flow interrupted by the gas phase is concentrated in the area occupied by the liquid, and consequently the battery voltage increases. Since plastic materials are typically hydrophobic in general, hydrophilicity essentially limits the choice of materials that can be used in the manufacture of planar devices to metals and metal oxides. For example, assuming a surface tension of caustic soda solution of 80 dyne / cm, the detected contact angle is 130 ° for PTFE, 120 ° for paraffin and 105 ° for polyethylene. In contrast, hydrophilic materials are known to be characterized by a contact angle of less than 90 °: metal surfaces are generally considered completely wettable, in fact they have a contact angle of caustic soda close to 0 °. From a theoretical standpoint, if complete wettability provides sufficient evidence that the planar element is completely occupied by the liquid, the metal or metal oxide that must be imparted to achieve this wettability is generally in the presence of boiling caustic soda containing trace amounts of peroxide. It is true that it is not stable enough. Insufficient stability here does not mean that the planar element is structurally destroyed within a relatively short operating time and, although slow, contaminates the membrane polymer and releases metal ions, which is unacceptable in practical terms. This means that performance can be degraded in time. The only metal believed to be without this action is silver, and its protective oxide, which is sufficiently impermeable and insoluble, degrades the release of metal ions to the extent that it does not damage the membrane. However, the production of planar devices using silver involves manufacturing costs that cannot be maintained in a wide range of industrial production. In addition, the use of other metals, such as stainless steel or nickel coated with a silver layer, does not provide a solution: in fact, because planar devices are in direct contact with the membrane, ion emission must be kept at extremely low levels, which is expensive. Only a thick layer obtainable by the procedure can be ensured. The importance of the planar element is indeed significantly higher than the current distributor (11 in FIG. 1), where the current distributor, which is significantly separated from the membrane surface, can consist of a thin, somewhat inexpensive layer of silver coated metal: due to the inevitable porosity of the silver layer This results in a certain amount of ions being released from the base metal, which is acceptable anyway, given the location of the current distributor relative to the membrane. As a result, from the teachings of the prior art, which recommends the use of hydrophilic materials for the manufacture of planar devices, perfluorinated plastic materials for planar devices, in particular characterized by high contact angles and thus high hydrophobicity, as described above. The fact of the present invention is particularly surprising that very satisfactory results are obtained by using plastic materials. Such plastic materials can be used by themselves or can be used, for example, as non-porous coatings applied to metallic planar elements made of carbon steel, stainless steel, nickel alloys with a thickness of 0.2 to 0.3 mm. Due to the hydrophobic characterizing the above materials, the oxygen bubbles trapped in the planar element structure must be intrinsically so that the voltage of the cell on which the planar element of the present invention is mounted is a European publication using planar elements with hydrophilic properties. It should be clearly higher than the typical voltage in an equivalent cell realized as described in patent EP 1033419 A1. Without limiting the invention in any way, as a result of the presence of the cathode in the immediate vicinity of the planar element, the expected negative effect may not occur due to the trapped bubbles: the cathode is in fact an electrode designed to absorb oxygen This action makes it possible to consume oxygen bubbles that may be present in the planar element in a less complete but certainly practical manner. Thus, if this theoretical hypothesis corresponds essentially to practicality, how to use the oxygen cathode-hydrophobic planar element combination according to the present invention is at least equivalent to that obtained by the oxygen cathode-hydrophilic planar element coupling described in the prior art. Understand if you will have the same results. This conclusion is appropriately supported by the experiments described in Example 1, which also demonstrates that the operational stability of the cells realized according to the invention is exceptionally high. This industrially relevant result appears to be derived from the chemical inertness of the perfluorinated material used as a preferred solution for the manufacture of planar devices. Chemical inertness prevents the release of components that may contaminate the membrane even after extended operating hours. The experiments described in the same Example 1 show that cells manufactured using hydrophilic planar elements made of metals according to the teachings of the prior art are adversely affected by the slow increase in cell voltage, which adversely affects silver layers, even thicknesses. Applying a uniform layer of silver to nonmetals does not work. The stability of the cell voltage only proves comparable to one of the cells of the invention when the planar element is made of pure silver. As a further advantage, perfluorinated plastics are characterized by good mechanical properties even in the temperature range of 80 to 90 ° C. which is typical for chlor-alkali electrolysis as is known. Thus, the planar element retains its shape as before, particularly with both the ratio of pore volume to occupied volume and porosity, and as illustrated above, an elastic support or membrane-plane element-oxygen cathode-current distributor assembly. It is not squeezed by the pressure difference used to bring it into close contact.

In the present invention, in a particularly preferred embodiment, oxygen (or oxygen containing gas) is supplied upwards to the bottom of the cell across the gap located at the rear of the oxygen cathode. This type of flow is considered to be of fundamental importance for the operational stability of the cell. To understand this, which is of fundamental importance for any industrial use, chlor-alkali electrolysis can be carried out with two different cell voltage values depending on whether the operation is carried out using a hydrogen emitting cathode or an oxygen cathode in a conventional manner. It should be noted that in the former case, the cell voltage is from about 2.5 V (low current density) to typically 3-3.3 V (current density of about 3 to 5 kA / m 2), while the latter is the cell voltage. Is in the range of about 1.6 to 2.2-2.6V and, as discussed earlier, significantly reduces energy consumption. Operating at 2.2-2.6V results in an inevitable inherent degradation of the activity of the catalyst inserted in the oxygen cathode and especially when the cell is operated at the highest current density (as required in the industry to minimize facility investment). If it does, it will work after several hours. As mentioned slightly above, cell voltage values of 2.5 V and above are especially true in areas where oxygen diffusion is impeded due to structural reasons (eg reduced porosity) or operational related reasons (eg fully localized flooding). Enable hydrogen release. Thus, in this situation, hydrogen release and oxygen consumption occur simultaneously, and hydrogen is inevitably mixed with oxygen to flow along the back of the cathode, forming a potentially explosive mixture. To avoid this risk, the percentage of hydrogen in oxygen should be monitored using a suitable sensor located at the outlet of the consumed oxygen (or oxygen containing gas). As found in the experiments based on supplying oxygen to the top of the cell in the present invention, the hydrogen content measured by the sensor tends to change significantly over time, and that of hydrogen as a function of operating conditions, in particular cell voltage and current density, This kind of measurement is totally unreliable because it is not associated with effective production. This unreliability is not an inherent characteristic of the sensor, but inherent in the mechanism of hydrogen accumulation in the gap retained in the flow of oxygen, representing a vertical development of 1 to 1.5 m. In fact, in the case of supplying oxygen to the top of the cell, hydrogen stratification occurs because the density of hydrogen is lower than that of oxygen: in short, the oxygen moves upward from the top to the bottom of the cell and rises to the top. The reverse current flows between hydrogen, which tends to be natural. This situation is dangerous because it easily forms hydrogen-rich pockets, especially in the low-circulation region, which makes the operation entirely unreliable. Periodic release of these pockets results in a variation in the percentage of hydrogen in released oxygen measured at the sensor.

In contrast, when hydrogen is supplied to the bottom of the cell, the result of the hydrogen flowing upwards coincides with the naturally rising hydrogen diffusion direction as mentioned above. In fact, hydrogen is introduced with oxygen-driven drag, which is very effective in preventing the formation of high hydrogen content pockets. The hypothesis of this behavior is actually evidenced by the data provided by the sensors, where the variation of the data is extremely limited (see Example 2). As a result, it is evident that supplying oxygen to the lower part of the cell equipped with the polarized cathode substantially increases the operational reliability, which is not simply a change of the prior art proposed to supply constantly from the top. Supply efficiency from the bottom can be further increased, for example, by using an inlet distributor and an outlet manifold of oxygen consisting of perforated tubes of the same length as the cell and located at the bottom and top of the cell. This type of design is shown in FIG. 2, which shows a front view of the cell of FIG. 1. The outlet 14 of oxygen (excessly supplied to the bottom of the cell) is connected to the internal manifold 18, the inlet 17 of oxygen is connected to the distributor 19, both of which are perforated tubes. Wherein the holes are preferably localized along the lower generatrix to facilitate the release of the condensed phase in some cases. A further refinement of the second aspect of the invention is made by the introduction of a suitable baffle to impart zigzag motion with high linear velocity to the oxygen flow, which helps to ensure uniform mixing with oxygen. Such an apparatus is shown in FIG. 3, where a baffle in the form of a horizontal strip is represented by 20. The gas flow direction is indicated by an arrow. The strip 20 has a few holes for facilitating the release of condensate separated on its surface, but is not shown in the figure. The entire portion of the hole is at least one order lower than the flow portion 21 existing between the distal portion of each strip and the outer edge of the cell.

Finally, the porous mass 23 can be advantageously inserted in the oxygen flow portion, preferably in part (22) of the embodiment of FIG. The porous mass is made of a highly porous material and is characterized by low resistance to gas flow, and a catalyst film capable of promoting hydrogen-oxygen recombination, such as platinum itself and an appropriate amount of platinum group metals such as palladium It is provided on the surface. Using this embodiment, not only the purpose of high gas mixing is achieved, but also the percentage of free hydrogen is reduced with it. Thus, it is possible to operate a cell equipped with an oxygen cathode under the most safe conditions relating to the amount of hydrogen generated and to form an unstable mixture even in the case of optimum mixing obtained according to the present invention.

The present invention has been cited with reference to chlorine-alkali electrolysis using oxygen cathodes, since this is a field of greater industrial relevance. As mentioned earlier, the present invention is also useful for the electrolysis of other processes, for example alkaline sulfates using hydrogen consuming anodes instead of conventional oxygen releasing anodes as described in US Pat. No. 4,561,945. In this case, a planar device with the above characteristics is sandwiched between the membrane and the hydrogen consuming anode, and allows the osmotic solution of sulfuric acid, optionally containing alkaline sulfate, to cross it. Also, in a manner very similar to that discussed in chlor-alkali electrolysis, hydrogen is supplied to the bottom of the cell to prevent oxygen from being locally enriched and anode byproducts from thickening in the stagnation zone. It is further applicable to alkaline fuel cells.

Example 1

The data for this example was obtained in the laboratory using several cells prepared according to the drawings of FIG. 1. In particular, each cell is 100 cm high and 10 cm wide and has two shells, an anode shell 2 and a cathode shell 7 each composed of titanium and nickel. In addition, the anode 3 supported on the rigid support 4 is a 1 mm thick titanium with a rectangular opening (diagonal = 4 mm x 8 mm) provided with an electrocatalyst film for chlorine release comprising titanium, iridium and ruthenium oxides. It consists of a flattened stretched mesh. The current distributor 11 consists of two parts, both of which comprise a 1 mm thick nickel drawn mesh with a rectangular opening (diagonal = 4 mm x 8 mm) and are fixed to the flexible support 12 and are small rectangular openings. It is welded to a 0.5 mm thick elongated second nickel mesh with (diagonal = 2 mm x 4 mm) and coated with 10 μm silver by zinc plating to ensure a number of suitable contact points with low electrical resistance to the oxygen cathode. Oxygen cathode is coated on one side with a layer of catalyst particles suitable for oxygen reduction (20% silver on Vulcan XC-72 carbon, for a total of 20 g / m 2 silver), mixed 1: 1 with tetrafluoroethylene particles. 80 mesh silver with a second layer of SAB carbon particles, mixed with tetrafluoroethylene particles in a 1: 1 weight, on another side (they are all sintered at 330 ° C. and have a final thickness of 0.5 mm). Wire net (0.2 mm diameter).

The planar element consists of a mesh formed by wires of 1 mm diameter and is superimposed and welded to form a rectangular opening (diagonal surface = 5 mm x 10 mm), each of polytetrafluoroethylene, high density polyethylene and polypropylene according to the present invention. , For comparison, is characterized by a two-sided continuous channel having a total thickness of about 2 mm prepared using nickel coated with 10 micron silver and pure silver according to the prior art.

When two shells, the anode shell 2 and the cathode shell 7 are fixed by suitable tie-rods, the anode current distributor 11 is pushed by the flexible support 12 and the anode-membrane (I A pressure of about 300 g / cm 2 is applied to the Pion N2010WX type) -planar element-oxygen cathode assembly.

About 20% sodium chloride solution flows in the cell's anode compartment, while a solution of 32% caustic soda is osmosis along the planar element at a constant flow rate of about 25 l / hr. Finally, pure oxygen is supplied to the top of the cell in a 10% excess than required for the reaction. Residual oxygen is released from the bottom of the cell through a suitable nozzle (17 in FIG. 1). At a temperature of 85 DEG C and a current regulated at 400 Amp (4000 A / m < 2 >), after a period of two days, the cell voltage actually detected regardless of the type of planar element was 2.30 V. However, after 42 days of operation, a cell equipped with a planar element consisting of polytetrafluoroethylene, high density polyethylene, polypropylene, and pure silver did not change, whereas 1 for a cell containing a nickel net coated with silver There was a tendency to increase by about 30 to 40 mV per day. At 2.5 V, the operation was stopped and the cell was inspected and the membrane showed brown coloration. Subsequent analysis showed that the membrane contained a significant amount of nickel. After about 50 days of operation, it was detected that the osmotic caustic soda flow rate was rapidly reduced, resulting in a rapid increase in the voltage of the cell containing the polypropylene planar element. Inspections performed after the opening and closing of the cell detected a significant compression of the planar element, possibly due to the enormous cracking of the polypropylene planar element caused by the corrosive action of traces of peroxide on the tertiary carbon atoms of the polymer chain. This degradation significantly reduced the available portion of caustic soda osmosis.

Testing conducted on day 72 of the operation of the cell equipped with the high density polyethylene planar device showed significant compression but no effect on the osmotic caustic soda flow rate.

The cell equipped with a planar element made of polytetrafluoroethylene and silver operated up to 224 days, the overall voltage increase was 20-30 mV, and the flow rate of caustic soda osmosis was not readily recognizable. On visual inspection, no abnormalities were seen in various parts. In subsequent analysis, the membrane appeared colorless and contained a small amount of calcium and magnesium and only negligible traces of nickel from the sodium chloride solution.

Example 2

The test of this example was carried out using the battery of Example 1 equipped with a planar element made of polytetrafluoroethylene according to the invention, except that the content of silver used as the electrocatalyst was limited to 10 g / m 2. Its purpose is, after initiation of the operation, to inevitable catalyst poisoning by impurities contained in the circulating caustic soda and / or catalyst changes caused by closure and subsequent initiation (dissolution / re-deposition phenomena that change catalyst particle size). As a result, it was to reproduce an insufficient activity state that increases with time.

Under the conditions shown in Example 1, a stable voltage of 2.6 V was detected, in particular when operated using a current density of 4000 Amp / m 2. In this situation, the analysis of hydrogen contained in the oxygen released in both the case of supplying oxygen from the bottom of the cell and the case of supplying oxygen from the top as described in the prior art was conducted.

In the former case, a fairly stable volume concentration of 470 ppm (maximum fluctuation about 50 ppm) was detected, while in the latter case a rather small concentration, for example about 50 ppm, fluctuating with a peak of 4000 to 5000 ppm was detected. This behavior can be explained on the assumption that hydrogen is produced mainly in a limited extension region, characterized by different oxygen diffusions, and that the downward flowing oxygen flow is opposed to the natural tendency of hydrogen to diffuse to the top of the cell. have. This situation can lead to the formation of hydrogen-rich pockets, which can potentially be very dangerous for operational safety. The local hydrogen enrichment potential is certainly eliminated by operating the upward flow of oxygen in accordance with the present invention to substantially coincide with the natural hydrogen diffusion direction.

Claims (29)

A cell suitable for an electrochemical process in which an anode compartment containing an anode and a cathode compartment containing a cathode are separated by an ion exchange membrane, The anode compartment, the cathode compartment or both are inserted into a current distributor connected to the wall of the counter compartment by a support, a gas diffusion electrode in contact with the surface of the current distributor, and a liquid reactant inserted between the ion exchange membrane and the gas diffusion electrode. It contains an assembly consisting of a porous planar element to be fed, wherein the liquid reactant osmulates into the interior of the porous planar element, And the porous planar element comprises a plastic material selected from the group of high density polyethylene and fluorinated plastics. The cell of claim 1, wherein the plastic material has a mutual contact angle of at least 90 ° and exhibits hydrophobic behavior with respect to the osmosis reactant into the interior of the device. The battery of claim 2 wherein the fluorinated plastic is PTFE, ECTFE, PFA or FEP. The battery according to any one of claims 1 to 3, wherein the plastic material is a coating applied to a metal material. The porous flat element according to any one of claims 1 to 3, wherein the porous flat element comprises a foam, a flat mesh consisting of crossing and overlapping wires, a flat mesh of knitted wire, a mesh with a controlled profile, a mattress of wire coils, stretched And a member selected from the group of mesh and sintered layered bodies or assemblies thereof. The current distributor according to any one of claims 1 to 3, wherein the current distributor is made by overlapping a first conductive foil provided with an opening with a second conductive foil provided with an opening having a smaller dimension than the first conductive foil, And the second conductive foil is in contact with the gas diffusion electrode. The battery of claim 6 wherein the first conductive foil is rigid. 7. A cell according to claim 6, wherein the first and second foils are selected from the group of elongated meshes, meshes of wires and perforated sheets. The battery according to any one of claims 1 to 3, wherein the support of the current distributor is elastic and the current distributor exerts a compressive force of the gas diffusion electrode and the porous planar element on the ion exchange membrane. The battery according to any one of claims 1 to 3, wherein the current distributor is divided into two or more parts. 10. A cell according to claim 9, wherein the elastic support comprises a conductive mattress made of coils or corrugated sheets. The anode or cathode compartment of claim 1, wherein the anode compartment or the cathode compartment comprises an assembly consisting of a current distributor, a gas diffusion electrode and a porous planar element, the support of the current distributor being rigid, the current distributor, the gas diffusion. And wherein the internal pressure of the compartment comprising the assembly consisting of an electrode and a porous planar element is lower than the pressure of the other compartment, and the ion exchange membrane compresses the assembly. The battery according to any one of claims 1 to 3, wherein the gas of the gas diffusion electrode is supplied to the lower part of the cell, and the consumed gas is discharged from the upper part of the cell. The battery according to claim 13, wherein the gas supplied to the lower portion is forced to zigzag by the baffle. The cell of claim 14, wherein the catalytic porous mass is inserted into the flow portion of the gas. An electrolytic cell of a modular arrangement of primary cells, characterized in that at least one of the elementary cells is a battery according to any one of claims 1 to 3. 17. The electrolytic cell of claim 16, wherein the base cell is electrically connected in a monopolar manner. The electrolyzer according to claim 16, wherein the base battery is electrically connected in bipolar. The battery according to any one of claims 1 to 3, which is used for electrolysis of alkaline halides. 20. A cell according to claim 19, wherein the cathode compartment comprises an oxygen feed gas diffusion cathode and a porous planar device fed with an osmotic alkaline hydroxide solution into the interior of the device. 21. The cell of claim 20, wherein oxygen is supplied at the bottom and released from the top. The battery according to any one of claims 1 to 3, which is used for the production of hydrogen peroxide. 23. The cell of claim 22, wherein the cathode compartment of the cell comprises an oxygen supply gas diffusion cathode and a porous planar device supplied with an osmotic alkaline hydroxide solution into the interior of the device. The cell of claim 23, wherein oxygen is supplied at the bottom and released from the top. The battery according to any one of claims 1 to 3, which is used for the electrolysis of alkaline sulphate. 27. The cell of claim 25, wherein the anode compartment comprises a hydrogen feed gas diffusion anode and a porous planar device supplied with a sulfuric acid solution that penetrates into the interior of the device, wherein the solution optionally comprises alkaline sulfate. The battery according to any one of claims 1 to 3, which is used for generation of electric current. 28. The cell of claim 27 wherein the cell is an alkaline fuel cell and wherein both the anode compartment and the cathode compartment contain an assembly consisting of a current distributor, a gas diffusion electrode and a porous planar element. delete

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